High frequency ultrasonic transducer and matching layer comprising cyanoacrylate

ABSTRACT

In one aspect, matching layers for an ultrasonic transducer stack having a matching layer comprising a matrix material loaded with a plurality of micron-sized and nano-sized particles. In another aspect, the matrix material is loaded with a plurality of heavy and light particles. In another aspect, an ultrasound transducer stack comprises a piezoelectric layer and at least one matching layer. In one aspect, the matching layer comprises a composite material comprising a matrix material loaded with a plurality of micron-sized and nano-sized particles. In a further aspect, the composite material can also comprise a matrix material loaded with a plurality of heavy and light particles. In a further aspect, a matching layer can also comprise cyanoacrylate.

BACKGROUND

Small animal imaging is an important field of research in many areasincluding preclinical pharmaceutical development, developmental biology,cardiac research, and molecular biology. Several small animal models arewidely used in these fields, the most prevalent being the mouse and therat. High frequency ultrasound has been widely used to image the mousemodel at frequencies from about 20 megahertz (MHz) to over 60 MHz. Therat model, however, is difficult to image at high frequencies incomparison to the mouse because the rat has highly attenuating andechogenic epidermal, dermal, and sub-dermal tissues.

SUMMARY

In one aspect, matching layers for an ultrasonic transducer stack havinga plurality of layers are provided. A matching layer can comprise acomposite material comprising a matrix material loaded with a pluralityof micron-sized and nano-sized particles. In another aspect, thecomposite material can also comprise a matrix material loaded with aplurality of heavy and light particles. In a further aspect, a matchinglayer can also comprise cyanoacrylate.

Also provided are ultrasound transducer stacks comprising a plurality oflayers, each layer having a top surface and an opposed bottom surface,wherein the plurality of layers includes a piezoelectric layer and atleast one matching layer. The matching layers can be positioned in thestack to overlie the top surface of the piezoelectric layer. Anexemplary stack can comprise a matching layer comprising a compositematerial loaded with a plurality of nano and micron sized particles, amatching layer with a plurality of heavy and light particles and amatching layer comprising cyanoacrylate.

Other systems, methods, and aspects and advantages of the invention willbe discussed with reference to the FIGS. and to the detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate certain aspects of the instantinvention and together with the description, serve to explain, withoutlimitation, the principles of the invention. Like reference charactersused therein indicate like parts throughout the several drawings.

FIG. 1 is a schematic diagram illustrating an exemplary transducer stackhaving a plurality of layers, and showing a plurality of matchinglayers.

FIG. 2 is a schematic diagram showing a cross section of an exemplarytransducer stack in the elevational dimension.

FIG. 3 is a block schematic diagram illustrating an exemplary transducerstack with electrical connections.

FIG. 4 is an enlarged schematic diagram illustrating layers of theexemplary transducer stack of FIG. 3, shown in exemplary scaleddimensions.

FIGS. 5A-5C are block diagrams illustrating exemplary methods offabricating an exemplary transducer stack.

DETAILED DESCRIPTION

The present invention can be understood more readily by reference to thefollowing detailed description, examples, drawings, and claims, andtheir previous and following description. However, before the presentdevices, systems, and/or methods are disclosed and described, it is tobe understood that this invention is not limited to the specificdevices, systems, and/or methods disclosed unless otherwise specified,as such can, of course, vary. It is also to be understood that theterminology used herein is for the purpose of describing particularaspects only and is not intended to be limiting.

The following description of the invention is provided as an enablingteaching of the invention in its best, currently known embodiment. Tothis end, those skilled in the relevant art will recognize andappreciate that many changes can be made to the various aspects of theinvention described herein, while still obtaining the beneficial resultsof the present invention. It will also be apparent that some of thedesired benefits of the present invention can be obtained by selectingsome of the features of the present invention without utilizing otherfeatures. Accordingly, those who work in the art will recognize thatmany modifications and adaptations to the present invention are possibleand can even be desirable in certain circumstances and are a part of thepresent invention. Thus, the following description is provided asillustrative of the principles of the present invention and not inlimitation thereof

As used herein, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to a “layer” includes aspects having two or more suchlayers unless the context clearly indicates otherwise.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another aspect includes from the one particular value and/orto the other particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

As used herein, the terms “optional” or “optionally” mean that thesubsequently described event or circumstance may or may not occur, andthat the description includes instances where said event or circumstanceoccurs and instances where it does not.

By a “subject” is meant an individual. The term subject includes smallor laboratory animals as well as primates, including humans. Alaboratory animal includes, but is not limited to, a rodent such as amouse or a rat. The term laboratory animal is also used interchangeablywith animal, small animal, small laboratory animal, or subject, whichincludes mice, rats, cats, dogs, fish, rabbits, guinea pigs, rodents,etc. The term laboratory animal does not denote a particular age or sex.Thus, adult and newborn animals, as well as fetuses (including embryos),whether male or female, are included.

The present invention may be understood more readily by reference to thefollowing detailed description of preferred embodiments of the inventionand the examples included therein and to the Figures and their previousand following description.

In one embodiment, the present invention is directed to matching layersfor ultrasonic transducer stacks having a plurality of layers.Ultrasound transducers, or transducer stacks, that are used for imagingutilize acoustic matching layers positioned between a piezoelectriclayer and a lens layer or face layer of the transducer. Thepiezoelectric layer typically has a high acoustic impedance (Z). Thesubject being imaged typically has a much lower acoustic impedance. Ifthe piezoelectric layer were pressed directly onto the subject, a greatdeal of the acoustic energy would be lost due to the impedance mismatchbetween the piezoelectric layer and the subject. In the ultrasoundimaging arts, matching layers with acoustic impedances between thepiezoelectric layer and the lens or face layer are introduced into thetransducer stack to provide a transition from the higher impedancepiezoelectric layer to the lower impedance subject.

Thus, the matching layers provided herein can be used in an ultrasonictransducer stack to accomplish an impedance transition from apiezoelectric layer to a lens or face layer. The exemplary matchinglayers can have varying acoustic impedances. One exemplary matchinglayer can have an acoustic impedance of between about 7.0 MegaRayles andabout 14.0 MegaRayles. Another exemplary matching layer can have anacoustic impedance of between about 3.0 MegaRayles and about 7.0MegaRayles. Yet another exemplary matching layer can have an acousticimpedance of between about 2.5 MegaRayles and about 2.8 MegaRayles. Oneskilled in the art will appreciate that each exemplary matching layercan be a ¼ wavelength matching layer.

An ultrasonic transducer stack can be used to generate, transmit andreceive ultrasound of high frequency (equal to or greater than 20megahertz). Exemplary ultrasonic transducer stacks comprise at least onedisclosed matching layer.

A schematic of such an exemplary transducer stack is shown in FIG. 1.FIG. 1 shows a transducer stack 100 having a lithium nibatepiezoelectric layer 102. The bottom surface of the piezoelectric layeroverlies the top surface of a backing layer 104. Above the top layer ofthe piezoelectric layer are an electrode layer 106, three exemplarymatching layers (108, 110 and 112), an epoxy bonding layer 114, and alens layer 116.

In this aspect, the matching layer 108 is a higher impedance matchinglayer that can have an acoustic impedance of between about 7.0MegaRayles and about 14.0 MegaRayles. In another aspect, the matchinglayer 108 can comprise nano and micron sized particles as describedbelow.

Above the upper surface of the matching layer 108 is the matching layer110, which has a lower impedance than the matching layer 108. Thematching layer 110 can have an acoustic impedance of between about 3.0MegaRayles and about 7.0 MegaRayles. In another aspect, the matchinglayer 110 can comprise light and heavy particles as described below.

The matching layer 112 has a lower impedance than the matching layer110. The matching layer 112 has an acoustic impedance of between about2.5 MegaRayles and about 2.8 MegaRayles. The matching layer 112 cancomprise cyanoacrylate as described below. The matching layer 112 can bebonded to the underlying matching layer 110 using a layer of epoxy 114.

The face layer of the exemplary transducer stack 100 comprises a lenslayer 116. The lens layer can comprise TPX as described below. The lenslayer 116 can be directly bonded to the matching layer 112. Thus, inthis exemplary transducer stack 100 the matching layers (108, 110, and112) accomplish an impedance transition from a piezoelectric layer 102to a lens layer 116.

A transducer stack, as exemplified herein, can be used to imagesubjects, or anatomical portions thereof, using high frequencyultrasound. The images produced can have a high resolution. In oneaspect, an ultrasonic transducer stack comprises a plurality of layers,each layer having a top surface and an opposed bottom surface. Inanother aspect, the plurality of layers includes a piezoelectric layerand at least one matching layer. When positioned in a transducer stack,the bottom surface of a given matching layer overlies the top surface ofthe piezoelectric layer.

A matching layer can comprise a composite material. In one aspect, thecomposite material can comprise a matrix material loaded with aplurality of micron-sized and nano-sized particles. In another aspect,the composite material can also comprise a matrix material loaded with aplurality of first heavy particles and a plurality of second lightparticles. In a further aspect, a matching layer can also comprisecyanoacrylate (CA).

Capturing of ultrasound data using an exemplary transducer stackcomprises generating ultrasound, transmitting ultrasound into thesubject, and receiving ultrasound reflected by the subject. A wide rangeof frequencies of ultrasound can be used to capture ultrasound data. Forexample, clinical frequency ultrasound (less than 20 MHz) or highfrequency ultrasound (equal to or greater than 20 MHz) can be used. Oneskilled in the art can readily determine what frequency to use based onfactors such as, for example, but not limited to, depth of imaging, ordesired resolution.

High frequency ultrasound may be desired when high resolution imaging isdesired and the structures to be imaged within the subject are not attoo great a depth. Thus, capturing ultrasound data can comprisetransmitting ultrasound having a frequency of at least 20 MHz into thesubject and receiving a portion of the transmitted ultrasound that isreflected by the subject. For example, a transducer having a centerfrequency of about 20 MHz, 30 MHz, 40 MHz, 50 MHz, 60 MHz or higher canbe used. In one exemplary preferred embodiment, the transducer can havea center frequency of about 20 MHz (for a design frequency of 25 MHz perthe general frequency example given herein below).

High frequency ultrasound transmission is often desirable for theimaging of small animals, where a high resolution may be achieved withan acceptable depth of penetration. The methods can therefore be used atclinical or high frequency on a small animal subject. Optionally, asnoted above, the small animal can be a rat or mouse.

The disclosed transducers can be operatively connected to an ultrasoundimaging system for the generation, transmission, receipt, and processingof ultrasound data. For example, ultrasound can be transmitted, receivedand processed using an ultrasonic scanning device that can supply anultrasonic signal of at least about 20 MHz to the highest practicalfrequency. Any ultrasound system or device capable of operating at 20MHz or above can be used.

The matching layers described herein can be used with other devicescapable of transmitting and receiving ultrasound at the desiredfrequencies. For example, ultrasound systems using arrayed transducerscan be used.

If a small animal subject is imaged, it can exemplarily be positioned ona platform with access to anesthetic equipment. Thus, the methods can beused with platforms and apparatus used in imaging small animalsincluding “rail guide” type platforms with maneuverable probe holderapparatuses. For example, the described systems can be used withmulti-rail imaging systems, and with small animal mount assemblies asdescribed in U.S. patent application Ser. No. 10/683,168, entitled“Integrated Multi-Rail Imaging System,” U.S. patent application Ser. No.10/053,748, entitled “Integrated Multi-Rail Imaging System,” U.S. patentapplication Ser. No. 10/683,870, now U.S. Pat. No. 6,851,392, issuedFeb. 8, 2005, entitled “Small Animal Mount Assembly,” and U.S. patentapplication Ser. No. 11/053,653, entitled “Small Animal Mount Assembly,”which are incorporated herein by reference.

Small animals can be anesthetized during imaging and vital physiologicalparameters such as heart rate and temperature can be monitored. Thus,the system can include means for acquiring ECG and temperature signalsfor processing and display. The system can also display physiologicalwaveforms such as an ECG, respiration or blood pressure waveform.

Also provided is the use of the described transducers or matching layersin a system for producing an ultrasound image using line-based imagereconstruction when a high frame rate is desired. One example of such asystem may have the following components as described in U.S. patentapplication Ser. No. 10/736,232, U.S. patent application publication20040236219, which is incorporated herein by reference. The disclosedsystem for producing an ultrasound image using line based imagereconstruction can provide an ultrasound image having an effective framerate in excess of 200 frames per second. The system comprises an ECGbased technique that enables high time resolution and allows theaccurate depiction of a rapidly moving structure, such as a heart, in asmall animal, such as a mouse, rat, rabbit, or other small animal, usingultrasound.

Many different organs of interest can be imaged including dynamic organshaving a lumen. For example, a heart, or a portion thereof, can beimaged using the methods and systems described herein. The methods andsystems are not limited to imaging the heart, however, and it iscontemplated that other organs or portions thereof, including otherportions of the cardiovascular system can be imaged.

Several small animal models are widely used in research, the mostprevalent being the mouse and the rat. High frequency ultrasound hasbeen used to image the mouse model with great effect at frequencies fromabout 20 MHz to over 60 MHz. However, the rat model has proved difficultto image at high frequencies in comparison to the mouse model due tohighly attenuating and echogenic epidermal, dermal, and sub-dermaltissues, which give rise to two main imaging obstacles. The first ishigh attenuation of the high frequency ultrasonic energy. The second isthe generation of multiple reflections that cause reverb imagingartifacts. Both of these obstacles can be mitigated by changing theoperational characteristics of the transducer used to image the rat.

Exemplary operational characteristics that overcome these imagingobstacles can comprise high sensitivity to overcome attenuation, amatched lens system to overcome reverb, and/or the use of a matchedattenuation layer between the transducer and the tissue to attenuatemultiple reflections. In addition, the transducer described for imagingthe rat can have a broad bandwidth, so as not to compromise axialresolution.

These three characteristics often come at the expense of each other andare generally regarded as engineering trade offs. In one example, moresensitivity generally comes at the expense of bandwidth. Similarly,better matching often involves lossy lens materials thus compromisingsensitivity and, if designed to be a ¼ wave match at the designfrequency, a reduction in bandwidth. The addition of a matchedattenuation layer also results in reduced sensitivity of the primarysignal level.

A transducer incorporating an acoustic match to water, sensitivity, andbroad bandwidth response is described herein and is useful for theimaging of a subject animal model. The transducer improves highfrequency ultrasonic imaging on rats and other small animal models.

In one aspect, to overcome the highly attenuative nature of the rattissues, a highly efficient transducer with very good sensitivity isdescribed herein. In a further aspect, the transducer is relativelybroadband with, for example, a −6 dB bandwidth of about 80% or greater.

In another embodiment, matching layers for an ultrasonic transducerstack having a plurality of layers are provided. In one aspect, thematching layer described can be a layer of an ultrasonic transducerstack including a piezoelectric layer. In alternative aspects, the stackcan comprise other layers, such as, for example and not meant to belimiting, a backing layer, other matching layers, a lens layer, a signalelectrode layer, a ground electrode layer, bonding layers and/or otherlayers known to those skilled in the art.

In one embodiment of the present invention, a matching layer comprises acomposite material. In this aspect, the composite material can comprisea matrix material loaded with a plurality of micron-sized and nano-sizedparticles. In an alternative aspect, the composite material can alsocomprise a matrix material loaded with a plurality of heavy and lightparticles. In another example, a matching layer can also comprise curedcyanoacrylate.

Exemplified herein are ultrasound transducer stacks that comprise aplurality of layers, each layer having a top surface and an opposedbottom surface. In one aspect, the plurality of layers includes apiezoelectric layer and at least one matching layer. In this aspect, thematching layers can be positioned in the stack such that theysubstantially overlie the top surface of the piezoelectric layer. Anexemplary stack can comprise a matching layer comprising a compositematerial loaded with a plurality of nano and micron sized particles, amatching layer with a plurality of heavy and light particles and amatching layer comprising cyanoacrylate.

Piezoelectric materials that can be used include, for example and notmeant to be limiting, ceramics, composite ceramic materials, and singlecrystals. For example, lithium niobate (LiNb) can be used for anexemplary single element mechanically scanned transducer. In anotherexample, 36 degree Y-Rotated Lithium Niobate is an exemplary materialfor the piezoelectric layer. LiNb has a highly efficient mechanicalcoupling characteristic (Kt of about 50%), and a very low dielectricconstant (∈_(r)=34) and can yield an efficient single element transducerthat may not use an additional electrical matching network. Further,Lithium Niobate (LiNb) has a high Q (in the region of 10,000) that canresult in a narrow band transducer. It is contemplated that the high Qcan be compensated for with a broad band matching structure and adamping backing system, which acts to reduce the Q of the transducer.

In a further aspect, a backing system can be used with the transducerstack and can be connected to and/or underlie the bottom sidepiezoelectric layer. If used, a backing layer accomplishes severalthings. First, it has an acoustic impedance that causes the transducerto resonate with the desired bandwidth. Secondly, it is highlyattenuating so that there are reduced or no internal reflections in thetransducer itself. Finally, the backing layer can be in operativecontact with the piezoelectric element.

In one exemplary aspect, the acoustic impedance of the backing layer ischosen as low as possible, relative to the Z of LiNb, to achieve highsensitivity while ensuring good bandwidth. For example, an acousticimpedance in the range of between about 5 MR to 7 MR gives a desirabletrade-off between sensitivity and bandwidth. For example, should ahigher bandwidth be desired, a backing impedance of between about 25 MRto 40 MR can be employed.

In regard to the attenuation of the backing layer, the higher theattenuation, the less backing thickness is required to eliminateinternal reflections. Also, a thinner backing layer results in thetransducer having less mass and volume.

In alternative aspects, the backing layer can be electricallyconductive, or it can be an insulator. However, whether a conductor orinsulator is used, the backing layer is operatively connected to thepiezoelectric layer. The conductive backing layer can result in a fastermanufacturing process, with a very narrow range of possible attenuationand acoustic impedances available. The non-conductive backing layerallows for a very wide range of damping and attenuation possibilities.

One exemplary backing layer is formed from Ablebond 16-1 conductiveepoxy. This backing layer material has an acoustic impedance of about6.7 MR and an attenuation of over about 100 dB/mm at 30 MHz. Inaddition, this exemplary conductive epoxy exhibits excellentconductivity at the bond line and makes an integral connection with thepiezoelectric layer.

In alternative aspects, to create very high bandwidth designs wherelower sensitivity is desired, metals can also be used to form thebacking layer such as, for example and not meant to be limiting, indium,tin, and alloys of indium.

In another aspect, a lens layer can also be used. For example, a lensthat is substantially acoustically matched to water can be used. Such alens can have a speed of sound either higher or lower than that ofwater, but sufficiently different from water so that a practicalcurvature can be realized to achieve a desired amount of focusing. Anexemplary lens material that can be used is polymethylpentene or TPX.This thermoplastic has an acoustic impedance of 1.8 MR and alongitudinal velocity of 2200 m/s. A convex lens (one with a speed ofsound lower than water) can also be used.

TPX is lossy compared to some other alternatives (5.7 dB/mm at 30 MHz),but provides an exceptionally good acoustic match to water and tissue.The primary challenge in using TPX is that it is very difficult to bondto other layers of the ultrasonic transducer stack. For example,Rexolite (a thermo set cross linked polystyrene) has a lower loss thanTPX having a loss of only about 1.1 dB/mm at 30 MHz, but has an acousticimpedance of 2.6 MR. Rexolite can be used where sensitivity is at apremium, and multiple reflections can be tolerated. By keeping the lensthin and the F-Number toward the higher end of the usual range, which istypically between about 2.5-3, the lossiness can be mitigated.

In various aspects, an exemplary at least one ¼ wavelength wave matchinglayer is used in the ultrasonic transducer stack. Conventionally, such ¼wavelength matching layers are also known simply as “matching layers.”It will be noted that the term “matching layer” is used throughout thedescription of the present invention and has the same meaning as ¼ waveor wavelength matching layer. The ¼ wave matching layer influences bothsensitivity and bandwidth.

At high frequencies, matching layers can be on the order of betweenabout 5.0 um to over 50.0 μm thick, and there is typically a lowtolerance for intermediate adhesive layers. For example, a layer of morethan 500 nm can be detrimental to the design, and anything over 1500 mmcan substantially negate the effect of the stack. As will be clear toone skilled in the art, for a ¼ wavelength layer, the thickness dependson the desired transmit frequency and the speed of sound in the layer.One of skill in the art could thus readily determine the appropriatethickness for a ¼ wavelength for a matching layer comprising heavy andlight particles through routine testing for the speed of sound of thecomposite and knowledge of the desired design frequency.

In one embodiment of the present invention, the exemplary lens layercomprises TPX that is connected to a cyanoacrylate matching layer, whichhas an acoustic impedance of approximately 3 MR. As described herein,the cyanoacrylate matching layer is bonded to the TPX lens layer forattachment to other layers of the stack.

In one exemplary embodiment, an approximately 10 MR matching layer isoverlaid on a matching layer having an impedance between about 4.5-5 MRto enhance the bandwidth and maintain excellent sensitivity. This canexemplarily be accomplished using two layers of tungsten doped epoxythat are sanded to a desired thickness using a vacuum sander. In afurther aspect, the lower impedance layer can also be doped with SiCnano-particles to prevent settling of the tungsten powder during curing.

As noted above, another matching layer can be the cyanoacrylate (CA)layer deposited thereon the TPX lens. The CA matching layer, bonded tothe lens layer, can be bonded with a layer of epoxy to the lowerimpedance matching layer, which is located in the stack underneath thebottom surface of the CA layer. In one aspect, the elevational thicknessof the epoxy layer is about 5 μm thickness or less. Due to the acousticsimilarity of the epoxy to the CA, at a thickness of a few microns (<5μm at 20 MHz) this layer is not significant to the performance of thestack. In one aspect, the epoxy can be, for example and not meant to belimiting, Epotek 301 epoxy. In another aspect, a rubber toughened CA(such as Loctite Black Max) can be used which can have a slightly loweracoustic impedance.

In alternative embodiments of the present invention, a matching layerfor an ultrasonic transducer stack comprising a plurality of layers cancomprise a composite material having a matrix material loaded with aplurality of micron-sized and nano-sized particles. In one aspect, thecomposite material forms the matching layer of the ultrasonic transducerstack. The matching layer can be a ¼ acoustic wavelength matching layer.

The particles can be of varying dimension within the respective nano andmicron size domains. In one preferred embodiment, the loaded particleshave a largest lengthwise or elongate dimension that is less than thethickness of the matching layer. For example, the micron-sized particleshave a largest lengthwise dimension that is about 5 μm and thenano-sized particles have a largest lengthwise dimension that is about800 nm wherein the matching layer thickness is larger than 5 μm. Oneskilled in the art will appreciate that the selected particles are assmall as possible without making it impossible to get to the desiredacoustic impedance. Nominally, in a matching layer, attenuation shouldbe kept to a minimum, and the particle size is much smaller than a wavelength. For example, a 5 μm particle in a 16.5 μm quarter wave layer isapproximately 1/13 of a wave length.

In one aspect, the micron-sized and nano-sized particles can comprise ahigh density metal. For example, the micron-sized and nano-sizedparticles can comprise tungsten, gold, platinum or a mixture thereof.Alternatively, high density ceramics, such as, for example, PZT, can beused if a non-conductive layer is desired.

In a further aspect, the matrix material can be a polymer. In onenon-limiting example, the polymer forming the matrix is an epoxy. Forexample, the epoxy can be a low viscosity, room temperature cure epoxywith a Tg above the maximum operating temperature of the transducer.Some non-limiting epoxy examples include Epotek® 301 and 302 (Epotek,Billerica, Mass.), Cotronics Duralco® 4461 (Cotronics, Brooklyn, N.Y.),West Systems Epoxies (West Systems, Bay City, Mich.) and variousAraldite® Epoxy combinations. Alternatively, the matching layer can alsobe a thermoplastic such as polymethylmethacrylate (PMMA), e.g., acrylic,plexiglas, Lucite, or polycarbonate (PC), e.g., Lexan.

In one exemplary aspect, the micron-sized and nano-sized particles canbe loaded in the matrix material in a ratio of between about 5:1 andabout 1:5 parts micron-sized particles to nano-sized particles byweight. For example, the micron-sized and nano-sized particles can beloaded in the matrix material in a ratio of between about 1:1 partmicron-sized particles to nano-sized particles by weight. In anotheraspect, as the desired acoustic impedance increases, the desiredpercentage of large particles can increase. For example, if a 10 MRmatching layer is desired, a 1:1 ratio can be used. In another example,for a 12 MR layer, a ratio of 2:1 or 3:1 micro:nano can be used.

In some exemplary aspects, the nano-sized particles and micron-sizedparticles of the matching layer comprise between about 10% and about 35%of the composite material on a volume basis. In other examples, thenano-sized particles and micron-sized particles can comprise betweenabout 25% and about 30% of the composite material on a volume basis. Inone preferred embodiment, the nano-sized particles and micron-sizedparticles comprise about 25% of the composite material on a volumebasis.

The matching layer having nano and micro particles can be designed witha desired acoustic impedance. For example, the acoustic impedance of thematching layer can be formed to be between about 7.0 MegaRayles (MR) and14.0 MegaRayles (MR). In one preferred aspect, the acoustic impedance ofthe matching layer is about 10 MR.

In various aspects, the matching layer can also vary in thickness. Asone skilled in the art will appreciate, the thickness to achieve a ¼wavelength matching layer will vary with the speed of sound in thematching layer and the frequency of the ultrasound passing through thematching layer. Thus, one of skill in the art could readily determinethe appropriate thickness of a matching layer based on the teachingsherein in combination with any desired ultrasound transmit frequency,including frequencies at and above 20 MHz. In other exemplary aspects,the speed of sound in the matching layer can be between about 1000meters per second (m/s) and 3000 m/s. Further, the elevational thicknessof a matching layer can be between about 4 μm and 30 μm.

In one embodiment, a process for producing the nano/micron particlematching layer comprises providing a matrix material, a plurality ofmicron-sized particles and a plurality of nano-sized particles. Thematrix material is loaded with a plurality of the micron-sized particlesand a plurality of the nano-sized particles to form a composite materialand the formed composite material is used to produce the matching layerof an ultrasonic transducer stack. In one aspect, the micron-sizedparticles and the nano-sized particles can comprise the same basematerial. Of course, it is contemplated that the micron-sized particlesand the nano-sized particles can also be comprised of different basematerials.

As shown in FIG. 1, the nano/micro particle matching layer can be usedas a matching layer in an exemplified ultrasonic transducer stack 100having a plurality of layers. As illustrated, the exemplary ultrasoundstack 100 comprises a plurality of layers, each layer having a topsurface and an opposed bottom surface. The plurality of layers includesa piezoelectric layer 102 and at least one matching layer. Of course,multiple matching layers (108, 110, and 112) can be used in thetransducer stack 100. The matching layer 108 comprises the ¼ wavelengthacoustic matching layer described above.

In some exemplary embodiments, the piezoelectric layer can generateultrasound at a center frequency of at least about 20 megahertz (MHz)for transmission through the first matching layer. Such high transmitcenter frequencies may be particularly desirable for imaging smallanimals, including rats. Thus, in one exemplary aspect, thepiezoelectric layer can generate ultrasound at a center frequency of atleast about 20 MHz, 25 MHz, 30 MHz, 35 MHz, 40 MHz, 45 MHz, 50 MHz, 55MHz, 60 MHz, 65 MHz, 70 MHz or higher for transmission through the firstmatching layer.

In another aspect, the piezoelectric layer can have an acousticimpedance of 20 MR or greater. As noted above, one exemplary type ofpiezoelectric layer that can be used comprises lithium niobate, whichhas an impedance of about 34 MR. In another example, the piezoelectriclayer can comprise PZT, which has an impedance of about 33-35 MR.

Of course, in addition to the nano/micro matching layer describedherein, other matching layers can be used. One exemplary matching layerof an ultrasonic transducer stack having a plurality of layers comprisesa composite material comprising a matrix material loaded with aplurality of first heavy particles and a plurality of second lightparticles. It should be noted, however, that the light and heavyparticles are not limited in size in this embodiment. Thus, the matchinglayer comprising the light and heavy particles can comprise a mixture ofnano and micro particles. For example, the light particles can be microsized or nano sized and the heavy particles can be micro sized or nanosized, any combination of which can be added to the matrix material.

The matching layer having heavy and light particles can comprise a ¼acoustic wavelength matching layer. In some non-limiting examples, thelight particles have a mass density of about 4.0 grams per cubiccentimeter (g/cc) or lower and the heavy particles have a mass densityof greater than about 4.0 g/cc. For example, the light particles canhave a mass density of between about 2.5 g/cc and about 4.0 g/cc. Theheavy particles can have a mass density of, for examples, 10.0 g/cc ormore.

For example, the first heavy particles can comprise micron-sized ornano-sized particles selected from the group consisting of tungstenparticles and lead zirconate titrate particles or a mixture thereof. Ina further example, the second light particles comprise micron-sized ornano-sized particles selected from the group consisting of siliconcarbite particles and alumina particles or a mixture thereof. Further,the density of the second light particles can be between about 100%-200%of the density of the final composite of the heavy particles and thematrix.

As described above, the heavy and light particles can vary in size. Invarious aspects, the heavy or light particles can be less than 1 micron.In a preferred embodiment, the loaded particles have a largestlengthwise or elongate dimension that is less than 1/50^(th) of a wavelength in the matching layer which they comprise. The heavy and lightparticles can be loaded in the matrix material, which can comprise apolymer such as, for example, an epoxy. In one example, the loadedplurality of particles can comprise at least about 11.0% of thecomposite material by volume. For example, the plurality of particlescan comprise between about 11.0% and about 20.0% of the compositematerial by volume. In a preferred embodiment, about 5.5% of thecomposite material by volume comprises a plurality of nano-sized heavyparticles and about 5.5% of the composite material by volume comprises aplurality of nano-sized light particles. In this preferred embodiment,as in other exemplary embodiments, the heavy particles can be tungstenparticles, PZT particles, gold particles, or platinum particles and thelight particles can be silicon carbite particles or alumina particles.

The acoustic impedance of the matching layer comprising heavy and lightparticles can vary. For example, the acoustic impedance of this layercan be between about 3.0 MegaRayles and 7.0 MegaRayles. In one exemplaryembodiment, the acoustic impedance is about 4.5 MR.

The thickness of the matching layer can also vary. One skilled in theart could thus readily determine the appropriate thickness for a ¼wavelength for a matching layer comprising heavy and light particlesthrough routine testing for the speed of sound and knowledge of thedesired transmit frequency. In one example, the matching layer can havea speed of sound of between about 1500 m/s and about 4500 m/s. In otherexamples, the speed of sound in the matching layer is between about 1800m/s and about 2500 m/s. In one preferred embodiment, the speed of soundin the matching layer is about 2100 m/s. In various aspects, it iscontemplated that the thickness of the exemplified matching layercomprising heavy and light particles can be between about 4.0 micronsand 30 microns. For example, for an exemplary 20 MHz center frequencytransducer, for a 25 MHz design frequency, comprising an exemplary mixof about 5.5% of the composite material by volume of nano-sized firstheavy particles and about 5.5% of the composite material by volume ofnano-sized second light particles, the matching layer is about 22.0microns thick in the elevational dimension of the ultrasonic transducerstack.

In one embodiment of the present invention, a process for producing thelight/heavy matching layer comprises providing a matrix material, aplurality of first heavy particles and a plurality of second lightparticles. In this aspect, the matrix material is loaded with aplurality of the first heavy particles and a plurality of the secondlight particles to form a composite material that is used as a matchinglayer of an ultrasonic transducer stack.

An ultrasound transducer stack can comprise a matching layer comprisinglight and heavy particles as described above. This matching layer cancomprise a lower impedance matching layer of a transducer stack thatalso comprises a higher impedance matching layer. In this aspect, thehigher impedance matching layer comprises nano and micro particles ofsimilar weight and or material as the lower impedance matching layer.

Thus, an exemplary stack 100 can comprise a plurality of layers, eachlayer having a top surface and an opposed bottom surface. The pluralityof layers can include a piezoelectric layer 102 and at least onematching layer. A matching layer 110 can comprise a composite materialcomprising a matrix material loaded with a plurality of first heavyparticles and a plurality of second light particles, wherein the bottomsurface of the matching layer 110 overlies the top surface of thepiezoelectric layer 102. The ultrasound transducer stack 100 can furthercomprise a matching layer 108 having a higher impedance than theimpedance of the matching layer 110, the matching layer 108 beingpositioned between the top surface of the piezoelectric layer 102 andthe bottom surface of the matching layer 110.

The piezoelectric layer can generate ultrasound at a center frequency ofat least about 20 megahertz (MHz) for transmission through one or morematching layer. For example, the piezoelectric layer can generateultrasound at a center frequency of at least about 25 MHz, 30 MHz, 35MHz, 40 MHz, 45 MHz, 50 MHz, 55 MHz, 60 MHz, 65 MHz, 70 MHz or higherfor transmission through one or more matching layer. The ultrasound canbe transmitted through the matching layer 108 and then through thematching layer 110.

Also provided herein is a backing layer 104 of an ultrasonic transducerstack 100 having a plurality of layers. The backing layer can comprise acomposite material having a matrix material loaded with a plurality ofmicron-sized and nano-sized particles. Also provided is a backing layer104 of an ultrasonic transducer stack 100 comprising a compositematerial having a matrix material loaded with a plurality of first heavyparticles and a plurality of second light particles.

EXPERIMENTAL

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how thecompounds, compositions, articles, devices and/or methods claimed hereinare made and evaluated, and are intended to be purely exemplary of theinvention and are not intended to limit the scope of what the inventorsregard as their invention. Efforts have been made to ensure accuracywith respect to numbers (e.g., amounts, temperature, etc.), but someerrors and deviations should be accounted for. Unless indicatedotherwise, parts are parts by weight, temperature is in ° C. or is atambient temperature, and pressure is at or near atmospheric.

Example 1 Fabrication of an Exemplary LiNb 20-25 MHz Transducer Stackwith a TPX Lens

FIG. 5 is a block diagram showing an exemplary method of fabricating anexemplary LiNb 20-25 MHz transducer with a TPX lens. The fabricationprocess is described below in three exemplary sections. First, thefabrication of matching layers and piezoelectric layer to form a stackis described. Second, the fabrication of the lens layer and acyanoacrylate matching layer is described. Finally, bonding of the lensand cyanoacrylate layer to the transducer stack is described.

Matching Layer and Piezoelectric Layer Fabrication

A LiNb Crystal is prepared for the piezoelectric layer as shown in block504. A 36 degree Y-Cut LiNb crystal is lapped to a thickness of 0.4lambda of the desired center frequency to compensate for mass loading.The crystal is plated with 3000 A of gold using a suitable means such asE-Beam deposition or Sputtering. As one skilled in the art willrecognize, typically a thin layer of Cr or Ni can be used to improveadhesion of the gold layer. The gold side of the LiNb crystal is cleanedwith Acetone. After cleaning, the crystal is placed in a clean placeuntil further handling.

A nano-particle and micro-particle loaded epoxy is prepared for ¼ wavematching layers. A high impedance matching layer, having an impedanceabove about 8 MR is prepared in block 506. Creating a loaded epoxycomposite with acoustic impedance over 8 MR is typically limited by themaximum volumetric ratio of powder that can be wetted by the epoxy.Achieving volumetric ratios of over 20% with particles small enough tobe compatible with a 25 MHz design is challenging due to the largesurface area to volume ratio of fine powders. With a 20% volumetriclimitation and using tungsten powder, it is difficult to createcomposites with acoustic impedances over about 8-9 MR. See, e.g., MarthaG. Grewe, T. R. Gururaha, Thomas R. Shrout, and Robert E. Newnham,“Acoustic Properties of Particle/Polymer Composites for UltrasonicTransducer Backing Applications,” IEEE Trans. on Ultrasonics,Ferroelectrics and Frequency Control, Vol. 37, No. 6, November 1990.

The use of a low viscosity epoxy (less than 1000 cps is preferred)allows for the greatest volumetric ratio of powder to be added to theepoxy before the mixture becomes too dry to use. An example of such anepoxy, known in the art, is Epotek 301. See, e.g., Haifeng Wang, TimRitter, Wenwu Cao, and K. Kirk Shung, “Passive Materials for HighFrequency Ultrasound Transducers”, SPIE Conf. on Ultrasonic TransducerEngineering, San Diego, Calif., February 1999, SPIE Vol. 3664. The epoxyis mixed completely before addition of any powders.

To create an exemplary 10 MegaRayles matching layer, a volumetric ratioof 25% powder in an epoxy matrix is used. To achieve this volumetricratio, relatively large 5 μm particles are used. However, such largeparticles are not compatible with the frequency of the transducer, asthey would allow for only 3 grains through the thickness of a matchinglayer. A sub-micron tungsten (W) powder mixed with the 5 μm powder to aratio of 1:1 is also used. This is effective over a range of at least1:6 to 2:1 by weight. The upper limit of density that can be achieved ina powder loaded epoxy is limited by the ability to wet the surface ofall the powder particles. As the particle size increases, the ratio ofthe volume of the powdered material to the surface area increaseslinearly. Thus, as the powder particle diameter increases, thevolumetric ratio of powder to epoxy that can be sufficiently wetted toresult in a void free mixture increases. However, as the particle sizeincreases, issues arise with settling and the increasingly significantinteraction of individual particles with the wavelength of theultrasonic energy.

To reduce attenuation in a matching layer and for predictable acousticimpedance of the matrix and powder as a compost material, the particlediameter can be small compared to the wavelength of the matrix (epoxy).For making a ¼ wave matching layer, the particle size can besufficiently small that the composite contains at least 15 or moreparticles through its thickness.

The exemplified use of a mixture of nano-particles with larger particlesallows for high density loaded powders with both high volumetricfractions of the loading powder and excellent control over settling.Settling is controlled by adjusting the amount of nano-particles tocontrol the viscosity and thixotropic index of the resulting paste. Inaddition to the advantages gained in the upper limit to the volumetricfraction attainable and the reduction of settling of larger particles,the nano-particles also provide that at any given cross section, the ¼wave layer (16.5 μm thick at 25 MHz for example) has a high number ofparticles and an even spatial distribution of powder particles (i.e.there are no large areas of epoxy between large particles as would bethe case with just large particles).

A mixture of large and small particles is preferred. The use of only thenano-particles results in an upper limit of less than 20% by volumebeing possible, while the exclusive use of 5.0 μm particles or even 2.0or 3.0 μm particles results in a highly attenuative matching layer witha poorly defined acoustic impedance due to large spaces betweenparticles compared to the wave length. An exemplary W doped Epoxypreparation comprises a mixed batch of 3:1 vol./vol. Epotek 301 epoxy to50%:50% 5 μm:<1 μm tungsten powder. This is a highly thixotropic pasteand is an 85% by weight tungsten mixture with a density of 5.7 g/cc anda volumetric fraction of 25% tungsten. Due to the small size of thenano-sized particles it has up to 50 particles through its thickness.For example, the mixture can be weighed out as follows: 0.5 g of mixed301 epoxy (0.1 g of hardener and 0.4 g of resin); 1.5 g of <1 μm Wpowder; 1.5 g of 5 μm W powder.

A medium acoustic impedance matching layer, having an impedance betweenabout 3.5 MR to 6 MR is fabricated as shown in block 514 by mixinglightweight particles with heavy particles. The creation of mediumacoustic impedance matching layers can use high volumetric fractions ofa single light particle to achieve mid and upper impedances in thisrange. However, using a single material, with a sufficiently smallparticle size to create medium acoustic impedance typically requires adifficult search for an appropriate density material available in asuitable powder grain size. High volumetric fractions make mixing,degassing, and spreading/application very difficult due to highviscosity and highly thixotropic pastes being created, which leads tomanufacturing issues. However, since the volumetric fraction of powderis typically kept above 11% to keep attenuation low, often a compromisemust be made between achieving the ideal acoustic impedance and idealphysical properties, or a new material must be sought and the processbegun again.

Thus, using light and heavy dopants together, a solution is possiblethat decouples the problem of achieving the desired acoustic impedancefrom the issues of viscosity, wetting, and thixotropic index. A heavymaterial is mixed to a volumetric fraction resulting in the desiredacoustic impedance, then light nano-particles are added until adesirable viscosity and thixotropic index are achieved (i.e., a pastethat is easily wetted, but will not flow all over or settle.) In oneexample, a commercially available light weight nano particle of SiC maybe used as the light weight particle. Several such particles can be usedincluding but not limited to SiC p=3.2 g/cc and alumina p=3.9 g/cc todecouple the problem of wetting, viscosity, and thixotropic index, fromthat of achieving a given acoustic impedance.

In this way, the acoustic impedance of the matching layer is controlledalmost exclusively by the volumetric fraction of the heavy powder, whichwould have significant settling issues at that fraction. However, theviscosity and consistency of the composite mixture and settling iscontrolled almost exclusively by the light weight powder. In thisaspect, the light weight powder is selected so that its density iswithin between about 100% to 200% of the density of the desiredcomposite density of the heavy powder-epoxy mixture.

For example, in an exemplary 25 MHz 4.5 MR matching layer, a 5.5%volumetric fraction of nano-particles of tungsten powder is used in asuitable room temperature cure low viscosity epoxy to achieve anacoustic impedance of between about 4.5 MR-5 MR. SiC nano-particles arethen added to achieve an exemplified 11% volumetric fraction.

This exemplified mixture is easy to work with, wets very well, and doesnot settle appreciably over a 24 hour cure period during which the epoxysets. Before addition of the SiC particles, the mixture settles outcompletely in seconds, however, after the addition of the SiC, themixture becomes thixotropic, and does not settle. The addition of SiCparticles minimally changes the acoustic impedance, but significantlychanges the viscosity and eliminates settling of the mixture. Anydesired acoustic impedance can be achieved in the medium range whilemaintaining a desirable working property without settling, and withoutneeding extremely high volumetric fractions using two readily obtainablepowdered materials. Also due to the size of the nano-particles,attenuation and scattering are kept to a minimum, making this anexceptional matching layer.

The W-Doped epoxy can be added to the stack as shown in block 508.Application of the matching layers requires careful attention, as airpockets in the matching layer can generally result in a malfunctioningstack. Air pockets near the interface of the piezoelectric crystal andthe powder loaded epoxy can be detrimental.

To prevent air pockets the loaded epoxy is spread evenly and air pocketsare forced to the surface of the mixture. Generally, the thixotropicpastes used to make matching layers are difficult to ‘flow’ onto a partand normally require agitation to flow like a liquid. Thus, a vibratingmanipulator (for example, an engraver with a piece of 22 gauge wireattached to it) can be used to spread the paste over the surface of thecrystal so that it flows and wets the entire surface. Further, thevibration encourages air pockets to rise to the surface of the paste,where they can be sanded away. This allows the use of highly thixotropicpastes that do not settle after they have been spread as desired.

In one example, under the microscope, an engraver tip modified with 22gauge hard temper copper wire L-Shaped tip is used to move thethixotropic paste smoothly over the surface for good wetting andpromotion of the rising of air pockets to the surface. Typically, theengraver tip is used over the entire surface of the aperture and is setto a low amplitude high frequency setting (about 7,200 spm). In oneaspect, the surface of the LiNb crystal is covered leaving a small partof the rim (about 0.25 mm) uncovered around the edge so that it can beused for a ground later. If epoxy is placed on the rim, it cannot becleaned effectively without removing the whole batch off the face of thetransducer.

The epoxy is allowed to cure at room temperature and post cure at anelevated temperature as shown in block 510. Room temperature cureepoxies are used so that significant shrinkage of the layers, which cancause warping of the piezoelectric crystal, does not occur. A high Tg ispreferred, so an epoxy is selected that is compatible with an elevatedtemperature post cure in an oven. The epoxy is allowed to cure at roomtemperature for about 18 hours or more (24 hours is preferred). Further,the cured epoxy is post cured in incubator for about 3 or more hours at65 degrees C.

The first matching layer is sanded and/or lapped as shown in block 510.Excess material is removed to achieve the ¼ wave matching layers.Several methods (such as lapping or sanding, etc.) can be used to removeexcess material leaving a ¼ wave thickness matching layer. A sandingsystem is used to sand the 1^(st) matching layer to a thickness oflambda/4 with c=1600 m/s (for the 25 MHz version this is 16-17 μmthick). Care is used to mount and measure the samples, as tolerances foran exemplary 20 MHz device for a design frequency of about 25 MHz are inthe 2-3 μm range, i.e., 16.5 μm+2 μm/−1 μm.

A second lower impedance layer is fabricated and applied as shown inblock 514. After completing a first matching layer, a second mediumacoustic impedance paste is applied to the first layer and the processof spreading, curing, and material removal is repeated to create thesecond ¼ wave layer. A batch is mixed as follows: W powder doped epoxyusing a 17:1 vol./vol. or (51% W by weight) mixture of Epotek 301 and <1μm Tungsten powder. 50% of the mass of the mixture in <1 μm SiC powderis added and mixed until a smooth paste is achieved. Any lumps in thepaste can be removed by mixing lightly in a mortar and pestle. Themixture may be weighted out as follows: 0.5 g of mixed 301 epoxy; 0.52 gof <1 μm W powder; 0.2 g of <1 μm SiC powder. The second layer isapplied as with the 1st, using the L-shaped tip in the engraver for evenspreading of the paste, and for good wetting. The mixture is allowed tocure at room temperature for about 18-24 hours, then post cured at 65degrees for about 3 hours or more.

The second matching layer is sanded and/or lapped as shown in block 516.The material is sanded to a thickness of Lambda/4 using c=2100 m/s,i.e., for the 25 MHz design frequency example this thickness is about18-19 μm thick+/−1 μm).

The support structure for the stack is prepared and the crystal isbonded to the support structure as shown in blocks 518 and 520. In oneaspect, the stack is positioned in a housing. A Ultem 1000(polyetherimide) insert can be used having an ID matching the desiredtransducer aperture and height and OD matching the specifications of thedesired Ti transducer housing so that the top of the insert is about 1.5mm below the rim of the Ti housing. The front face of the insert iscleaned and inspected so that it is clean and free of burrs or flashing.For example, the insert can be cleaned with the ultrasonic cleaner anddetergent initially and can be cleaned just before use with isopropylalcohol.

A small quantity of a suitable low temp cure medium viscosity epoxy(e.g., Loctite E-20 HP) is prepared and is applied to the cleansed frontface of the insert in a very thin coat using a sponge tipped swab. Theepoxy is not applied so heavily as to form a meniscus across the wallthickness of the insert. In operation, the crystal is placed, with thestack side up, onto the epoxy covered face of the insert and is centeredon the insert. In one example, the lithium niobate crystal is applied tothe insert by using a vacuum pick-up tool. A small amount of force isapplied to push the crystal onto the face of the insert, which causesthe epoxy to flow toward the edges of the underlying insert. Thecentered crystal is placed into an incubator for curing at about 40degrees C. for about 3 hours.

After curing, the insert/crystal assembly is inspected to ensure thatthe crystal is centered and completely adhered to the insert.Subsequently, the periphery of the insert/crystal assembly is cleaned.The non-increased diameter of the Ultem insert is checked by sliding thepart into an exemplary housing.

At this stage, one skilled in the art will appreciate that the acousticstack can be housed onto a suitable support structure and a rearelectrode and suitably attenuating backing material can be applied asshown in blocks 522 and 524. These functions can be combined by usingconductive epoxy as a backing layer.

In one example, the exemplified insert/crystal assembly is normallyplaced in a clean smooth work surface crystal layer down. The backcavity is filled with Ablebond 16-1 silver conductive epoxy. The cavityis filled by first placing a dollop of epoxy in the center of the cavityusing an epoxy syringe and applicator tip. The epoxy is applied to theentire back face of the crystal making sure not to allow any air pocketsto be trapped in the backing material. The epoxy syringe is used tocontinue filling with the tip of the applicator below the surface of theepoxy so that no air pockets are created in the epoxy when filling. Thecavity is filled until the conductive epoxy is about 0.5 mm below therim of the insert.

The backed stack, called a “pill,” can now be housed in a suitablehousing depending on the application that the device is intended for,taking into consideration, weight budgets, temperature, and RFshielding, etc. and is sealed as shown in blocks 526 and 528.

The pill is placed onto the Housing fixture with the uncured epoxy up.Several small dots of Loctite E-20 HP are applied to the back rim of thepill such that the dots do not flow together. Next, a completed housingis laid over the pill until the pill makes contact with the back of thehousing. Subsequently, the fixture clamp is placed over the housing tohold it in place, and allowed to cure at room temperature for about 18hours or more. The assembly is then placed into an incubator at 65degrees for about 3 hours for post curing. Typically, the face of thepill is oriented parallel to the floor and pointing down to help preventthe backing layer from flowing inside the housing.

Next, a sealing layer can be applied. In this example, when the epoxy isfully cured, a bead of epoxy is applied around the perimeter of thecrystal so that a continuous smooth surface exists between the crystaland the Ti housing. Here, a very small amount of Epotek 301 epoxy isplaced around the perimeter of the LiNb crystal and the inside diameterof the housing. The surface of the crystal is about 1.25 mm belowchamfer on the Ti housing in order to desirably achieve a negativemeniscus of epoxy between the crystal and the Ti housing. Both thecrystal and the Ti are clean and free of epoxy on their surfaces so thatgold can be sputtered onto both surfaces. In operation, a highmagnification (about 20 times and preferably more) is used to place theepoxy bead. In one example, the epoxy can be applied at three locationsaround the perimeter and allowed to run around the perimeter usinggravity and capillary action. In a further example, a piece of finegauge wire (26 gauge or so) can be attached to the end of a sharp Q-tipto improve control to help the glue to flow around the entire perimeter.The resultant construct is allowed to cure at room temperature for about12-18 hours. The sealing procedure can be repeated and the construct isallowed to cure at room temp for about 18 hours then is incubated at 65degrees for a post cure of about 3 hours. After the epoxy has cured, therim of the Ti housing is inspected for epoxy. Any epoxy found on the rimis removed.

As one will appreciate, a ground connection to a signal return path ismade to the exposed gold rim of the acoustic stack as shown in block530. Again, one skilled in the art will recognize several methods ofattaching such an electrode including sputtering, conductive inks andepoxies, direct mechanical contact with a good conductor, etc.

In one example of the ultrasonic stack, the stack, as formed above, canthen be bonded to a third low impedance matching layer and a lens. Awell-matched lens is selected. As noted above, in one preferredembodiment, the lens is closely matched to water in terms of acousticimpedance to provide for a reduction of reverberation artifacts.Furthermore, the lens material can be low in attenuation and have aspeed of sound sufficiently different from water so as to be able tocreate a focusing effect without extreme curvature. There are manyexemplary materials that have been used to form the lens for ultrasonictransducers. Furthermore, many transducer designs make use of curedpiezoelectric elements, or array structures as alternatives to lenses.

For the purposes of imaging rats, the reverb artifact is a significantconcern in the design of a lens. While other materials exist that havelower loss at high frequency, or more refractive power, TPX wasidentified as a preferred lens material that has an acoustic match towater. TPX is a member of the polyolefin family and has a close acousticimpedance match to water and tissue compared to most plastics. TPX hasan acoustic impedance of between about 1.78 MR to 1.85 MR. See, e.g.,Alan R. Selfridge, “Approximate Material Properties in IsotropicMaterials,” IEE Trans. Sonics and Ultrasonics, Vol. SU-32, No. 3, May1985. The impedance of water is Z=1.5 MR.

It is well known that TPX is difficult to bond with epoxies and mostother adhesives. The most common use of TPX in industry is as a releasefilm. That is, TPX is recognized as a material that most things will notstick to. While some surface preparation techniques can be used toenhance bond strength, in fact, even with adhesion promoters andprimers, or coronal etching TPX is often not bondable in a demandingapplication and must be jointed either mechanically or heat sealed orwelded. See, Timothy Ritter, K. Kirk Shung, Xuecang Geng, Pat Lopath,Richard Tutwiler, and Thomas Shrout,” Proceedings of SPIE—Volume 3664Medical Imaging 1999: Ultrasonic Transducer Engineering, K. Kirk Shung,Editor, June 1999, pp. 67-75.

The bondline between a lens and an acoustic stack in an imagingtransducer can experience temperature swings of over 40 degrees Celsiusin a few minutes, and rapid cooling routinely during use. In addition,it is constantly bombarded with ultrasonic energy. Weakness in bondinggenerally results in delamination and dead spots in the transducer. Forthis reason, high quality bond lines are desired for transducerconstruction.

For the exemplary high frequency ultrasonic transducers, the lensremains in contact with the stack at all times. Even a thin disbond canresult in a dead spot in the transducer.

In one exemplified example, cyanoacrylate (CA) adhesive systems arebonded to the TPX lens. With the use of a suitable primer such as, forexample and not meant to be limiting, Verik AC77, some toluene basedprimers, and the like, CA can form a robust bond to TPX. However,because of cyanoacrylate's curing characteristics being very sudden anddependent on substrate and environmental conditions, CA has not beenused in transducer stacks. The glue can snap cure suddenly at the verythin bond lines used in making a high frequency ultrasonic stack. Theaddition of bondline spacers is often not used with CA adhesives as theycause instant curing of the material due to their large surface areas.CA cannot be powder loaded for the same reason, further making it anunlikely candidate for transducer manufacturing. Moreover, acousticproperties of CA have not been available in the literature, as thematerial cannot be cured in a thick enough section for standard testing.Furthermore, material properties that may be used to acoustically modelCA have been unavailable for similar reasons.

Despite the lack of art and drawbacks noted in the art, CA can be curedover a narrow range of thickness. The range over which CA can be curedcan be used for forming ¼ wave matching layers for frequencies from 5MHz up to over 60 MHz. The acoustic properties of CA were determined byusing it as a matching layer and were correlated to a model usingPiezoCAD software based on the KLM model available from Sonic Concepts,Woodinville, Wash. USA. The results showed that CA can be used as amatching layer with acoustic impedance of between about 2.5 MR −2.8 MR.

One exemplary method to bond TPX to a ¼ wave matching system makes useof CA. The bond between the TPX layer and the CA layer was tested andresults indicated that the interface between the CA layer and TPX layeris at least as strong as the TPX itself. However, for reasons statedabove, it is not suitable for bonding directly to the stack, as the bondthickness cannot be controlled, and voids and misalignment are likely,which can ruin an almost complete transponder stack at the last stagesof assembly.

In one exemplary process, a ¼ wave thick cured layer of CA is created onthe back (flat side) of the TPX lens layer, and the formed construct isthen bonded to the upper surface of the previously formed stack with atraditional epoxy. The epoxy bonds readily to the CA layer, and the CAlayer, which is, in turn, bonded to the TPX lens layer. Thus the CAlayer forms a ¼ wave matching layer from the top of the stack to the TPXlens layer. In the case of the exemplary stack, the top of the stack hasan acoustic impedance of 4.5 MR, with the CA having Z=2.5−2.8, and theTPX lens layer Z=1.8. This yields a transducer with a −6 dB bandwidth of85%-90% and a two way insertion loss of between approximately −41 dB to−42 dB at 25 MHz rel V/V.

In one example, a quantity of CA glue, such as, for example, Verik PR40,is coated onto the back of the TPX lens by means of an aluminum foilrelease layer and wire spacers as shown in blocks 534 and 536. Inoperation, the release film is placed onto a flat surface (preferably avacuum plate so that the foil is flat). In one aspect, the aluminum foilis clean and free of oil and moisture. Wires are laid in a pattern ontothe release film. In one aspect, the wires are positioned in a radialpattern so that the wires do not meet at their common vertex. Thesewires are used as spacers for the layer of CA. For the exemplary 25 MHzstack, a wire having a diameter of about 25 μm is used to provide form aCA layer having a resultant thickness about 23 μm-25 μm. This isslightly thicker than the desired ¼ wave thickness for CA, which wouldbe between about 21 to 23 μm for a 25 MHz design frequency transducer.In this aspect, it is estimated that the longitudinal velocity in CA isbetween about 2100 m/s to 2200 m/s. The 1-3 μm surplus material isremoved during a later abrasion process which also acts to prepare thesurface of the CA layer to improve adhesion of the epoxy used to gluethe lens layer having the adhered CA layer to the underlying stack.

In operation, the back face of the TPX lens layer is abraded with asuitable grit SiC sandpaper to improve the surface for adhesion and istreated with a suitable CA polyolefin primer, such as, for example,Verik AC77, toluene based primers, and the like, as shown in block 532.The back of the lens is then covered with a generous amount of CA sothat substantially the entire surface is wetted as shown in block 536.CA will not cure quickly when allowed to form a relatively thickmeniscus on the back of the lens, and so the generous amount of CAprovides sufficient time to place the lens onto the prepared releasefilm/wire arrangement.

Subsequently, the lens layer, with the applied CA, is placed onto therelease film and pressed lightly so that the lens “sandwiches” the wiresbetween the underlying release film and the lens layer as shown in block538. After curing for about two hours, the lens and the attached releasefilm is removed from the vacuum plate and the release film is peeled offthe lens layer as shown in block 542. Next, the CA layer is allowed tocure for about 24 hours, as shown in block 540, at which time 1-3 μm ofCA are then removed with SiC sand paper to abrade the surface of thenewly formed CA layer in preparation for bonding to the stack as shownin block 544.

The composite formed lens layer/CA layer is bonded to the transducerstack as shown in blocks 548, 550 and 552. In one example, a suitablelow viscosity RT cure epoxy, such as Epotek 301, with an acousticimpedance value close to that of the CA layer is used to glue thecomposite lens layer/CA layer to the underlying stack. In one aspect,the lens is held in place using a fixture to maintain pressure of atleast about 100 kPa during curing of the epoxy. An extended post cure atan elevated temperature using an external heat source is used for curingepoxy at such a thin bondline, which ensures the production of abondline of less than 5 μm, and preferably from 1 μm-3 μm. One wouldappreciate that, since the acoustic impedance of the epoxy is similar tothat of the CA, there is little, if any, contribution to the stack bythis bond layer.

The above exemplary methods can be used to create a transducer stackwherein the piezoelectric layer has a center frequency range of fromabout 5 MHz to about 60 MHz or higher. Such a transducer can be used toimage small animals, including the rat, using high transmit frequencies(greater than or equal to 20 MHz).

Example 2 General High Frequency Design for an Exemplified Broadband(85-95%—6 dB Bandwidth) LiNb Transducer with TPX Lens

Table 1 shows the different layers that make up an exemplary transducerstack. The stack design can be used for transducers with a centerfrequency from about 20 MHz to over 60 MHz.

The design center frequency f_(D) is chosen to be higher than thedesired operating center frequency of the device f_(O) to compensate formass loading which reduces the center frequency of a device. f_(D) isthe frequency that the device would operate at in air with no lens andair backing. For this exemplary design, f_(D) is chosen to beapproximately 1.15 to 1.25 times the desired center frequency of thefinal transducer. For example, for a 20 MHz device, one would choosef_(D)=˜23-25 MHz for the purpose of the relationships shown in Table 1below.

TABLE 1 TPX Lens CA (cyanoacrylate) ¼ Wave Matching layer${{Thickness} = \frac{c_{L}}{4\; f_{D}}},{{where}\mspace{14mu} c_{L}\mspace{14mu} {is}\mspace{14mu} {the}\mspace{14mu} {longitudinal}\mspace{14mu} {velocity}\mspace{14mu} {of}\mspace{14mu} {the}}$relevant layer Thin layer of Low Viscosity RTC Epoxy (Typical: Epotek301, Duralco 4461) ¼ Wave Matching Layer of Moderate Impedance SiCnano-particles and Tungsten nano-particles doped Epoxy${Thickness} = \frac{c_{L}}{4f_{D}}$ ¼ Wave Matching layer of HighImpedance Tungsten (5 μm particles mixed with tungsten nano-particlesdoped Epoxy ${Thickness} = \frac{c_{L}}{4f_{D}}$${{Thickness} = {k\frac{c_{L}}{f_{D}}}},{{{where}\mspace{14mu} k} = 0.4}$Conductive Epoxy Backing with Z = 5 MR − 7 MR

Throughout this application, various publications are referenced. Thedisclosures of these publications in their entireties are herebyincorporated by reference into this application in order to more fullydescribe the state of the art to which this invention pertains.

The foregoing detailed description has been given for understandingexemplary implementations of the invention only and no unnecessarylimitations should be understood therefrom as modifications will beobvious to those skilled in the art without departing from the scope ofthe appended claims and their equivalents.

The preceding description of the invention is provided as an enablingteaching of the invention in its best, currently known embodiment. Tothis end, those skilled in the relevant art will recognize andappreciate that many changes can be made to the various aspects of theinvention described herein, while still obtaining the beneficial resultsof the present invention. It will also be apparent that some of thedesired benefits of the present invention can be obtained by selectingsome of the features of the present invention without utilizing otherfeatures. The corresponding structures, materials, acts, and equivalentsof all means or step plus function elements in the claims below areintended to include any structure, material, or acts for performing thefunctions in combination with other claimed elements as specificallyclaimed.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatan order be inferred, in any respect. This holds for any possiblenon-express basis for interpretation, including: matters of logic withrespect to arrangement of steps or operational flow; plain meaningderived from grammatical organization or punctuation; and the number ortype of embodiments described in the specification.

Accordingly, those who work in the art will recognize that manymodifications and adaptations to the present invention are possible andcan even be desirable in certain circumstances and are a part of thepresent invention. Other embodiments of the invention will be apparentto those skilled in the art from consideration of the specification andpractice of the invention disclosed herein. Thus, the precedingdescription is provided as illustrative of the principles of the presentinvention and not in limitation thereof. It is intended that thespecification and examples be considered as exemplary only, with a truescope and spirit of the invention being indicated by the followingclaims.

1. An ultrasonic transducer stack, comprising: a plurality of layerscomprising: a piezoelectric layer; at least one matching layer thatcomprises a first matching layer, which comprises cyanoacrylate; and alens layer comprising TPX, wherein each layer of the plurality of layershas a top surface and an opposed bottom surface, wherein the firstmatching layer is connected to and underlies the bottom surface of thelens layer, and wherein the piezoelectric layer underlies the bottomsurface of the matching layer.
 2. The ultrasonic transducer stack ofclaim 1, wherein the first matching layer is about a ¼ acousticwavelength matching layer.
 3. The ultrasonic transducer stack of claim2, wherein the acoustic impedance of the first matching layer is betweenabout 2.0 MegaRayles (MR) to about 3.5 MegaRayles (MR).
 4. Theultrasonic transducer stack of claim 2, wherein the acoustic impedanceof the first matching layer is between about 2.5 MegaRayles (MR) toabout 2.8 MegaRayles (MR).
 5. The ultrasonic transducer stack of claim2, wherein the acoustic impedance of the lens layer is about 1.8MegaRayles (MR).
 6. The ultrasonic transducer stack of claim 1, whereinthe acoustic impedance of the lens is substantially the same as theacoustic impedance of water.
 7. The ultrasonic transducer stack of claim1, wherein the piezoelectric layer can generate ultrasound at afrequency of at least about 20 megahertz (MHz).
 8. The ultrasonictransducer stack of claim 7, wherein the piezoelectric layer cangenerate ultrasound at a frequency of about 20 MHz, 25 MHz, 30 MHz, 35MHz, 40 MHz, 45 MHz, 50 MHz, 55 MHz, 60 MHz, or higher for transmissionthrough the first matching layer and then through the lens layer.
 9. Theultrasonic transducer stack of claim 1, further comprising a secondmatching layer located between the top surface of the piezoelectriclayer and the bottom surface of the first matching layer.
 10. Theultrasonic transducer stack of claim 9, wherein the top surface of thesecond matching layer is bonded to the bottom surface of the firstmatching layer using a bonding material.
 11. The ultrasonic transducerstack of claim 10, wherein the bonding material forms a bondline layerbetween the first matching layer and the second matching layer.
 12. Theultrasonic transducer stack of claim 11, wherein the bondline layer hasan elevational thickness of less than about 5 microns.
 13. Theultrasonic transducer stack of claim 11, wherein the bondline layer hasan elevational thickness of between about 1 micron to about 5 microns.14. The ultrasonic transducer stack of claim 11, wherein the bondlinelayer has an elevational thickness of between about 1 micron to about 3microns.